Gas turbine engines commonly utilize variable vane assemblies to control the flow of a fluid, usually air or combustion products, through various compression and expansion stages of the engine. Typically, they comprise Inlet Guide Vanes (IGVs) or Stator Vanes (SVs) disposed within the flow passages of the engine adjacent to rotor blade assemblies, usually in the compressor stages or fans of the engine although variable stator vanes may also be used in power turbines. Air passing between the vanes is directed at an appropriate angle of incidence for the succeeding rotating blades.
Each vane in a variable vane assembly is rotatably mounted about its longitudinal axis within the flow path of a compressor or turbine. The vane is connected at its radially outer end to a lever which, in turn, is pivotally connected to a unison ring. The unison ring is mounted on carriers so that it is rotatable about its central axis, which coincides with the engine axis.
The unison ring is rotated by means of one or more actuators, acting on the ring. The actuators exert a tangential load on the unison ring causing the ring to rotate about its central axis. Rotation of the unison ring actuates each of the levers causing the vanes to rotate, in unison, about their respective longitudinal axes. The vanes can thus be adjusted in order to control the flow conditions within the respective compressor or turbine stages.
It is known that when a unison ring is not properly centralized around the engine casing, it may impart vane angle errors within the variable vane assembly. Unison ring decentralization may be caused by gravity, assembly loads, the number of actuators, warpage, or a variety of operating conditions. In addition, the engine casing often experiences thermal expansion during operation. This thermal expansion can vary the gap between the unison ring and the engine casing. Attempts to properly center the unison ring on the engine casing must accommodate the varying tolerances caused by such thermal expansion.
Overcoming these concerns would be helpful, could improve vane angle accuracy, and could minimize variations caused by thermal expansion.